NEOWISE Observations of Near-Earth Objects: Preliminary Results (1109.6400v1)

Abstract: With the NEOWISE portion of the \emph{Wide-field Infrared Survey Explorer} (WISE) project, we have carried out a highly uniform survey of the near-Earth object (NEO) population at thermal infrared wavelengths ranging from 3 to 22 $\mu$m, allowing us to refine estimates of their numbers, sizes, and albedos. The NEOWISE survey detected NEOs the same way whether they were previously known or not, subject to the availability of ground-based follow-up observations, resulting in the discovery of more than 130 new NEOs. The survey's uniformity in sensitivity, observing cadence, and image quality have permitted extrapolation of the 428 near-Earth asteroids (NEAs) detected by NEOWISE during the fully cryogenic portion of the WISE mission to the larger population. We find that there are 981$\pm$19 NEAs larger than 1 km and 20,500$\pm$3000 NEAs larger than 100 m. We show that the Spaceguard goal of detecting 90% of all 1 km NEAs has been met, and that the cumulative size distribution is best represented by a broken power law with a slope of 1.32$\pm$0.14 below 1.5 km. This power law slope produces $\sim13,200\pm$1,900 NEAs with $D>$140 m. Although previous studies predict another break in the cumulative size distribution below $D\sim$50-100 m, resulting in an increase in the number of NEOs in this size range and smaller, we did not detect enough objects to comment on this increase. The overall number for the NEA population between 100-1000 m are lower than previous estimates. The numbers of near-Earth comets will be the subject of future work.

• The paper refines NEA statistics by analyzing thermal IR measurements to estimate populations, sizes, and albedos.
• It applies the NEATM model to both cryogenic and post-cryogenic data, ensuring consistent sensitivity across observations.
• The findings improve our understanding of asteroid hazards, supporting advances in planetary defense and exploration.

NEOWISE Observations of Near-Earth Objects: An Overview

The paper "NEOWISE Observations of Near-Earth Objects: Preliminary Results" presents an extensive analysis of data gathered by the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE), which is part of the Wide-field Infrared Survey Explorer (WISE) mission. The significant contribution of this research lies in its systematic survey of near-Earth objects (NEOs) at thermal infrared wavelengths, providing refined estimates of their numbers, sizes, and albedos. This research offers a pronounced confidence in the statistical understanding of the NEO population and insight into their potential hazards and exploration opportunities.

Key Findings

• Population and Size Estimates: The paper estimates that there are approximately 981 ± 19 near-Earth asteroids (NEAs) larger than 1 km, and around 20,500 ± 3000 NEAs larger than 100 m. These figures suggest that the Spaceguard goal of detecting 90% of all NEAs larger than 1 km has been effectively met.
• Size Distribution: The cumulative size distribution of these NEAs is best illustrated by a broken power law with a slope of 1.32 ± 0.14 below 1.5 km, generating a population estimate of ∼13,200 ± 1,900 NEAs with diameters greater than 140 meters.
• Detection and Observational Methods: NEOWISE detected NEOs irrespective of their prior recognition, subject to available ground-based follow-up observations. It achieved a highly uniform sensitivity across observations facilitated by the thermal infrared wavelengths ranging from 3 to 22 μm.
• Albedo Insights: The paper identifies that visible albedos (reflectivity) do not strongly correlate with diameter, a conclusion that suggests any previously reported linkages between albedo and size could be more indicative of observational biases rather than intrinsic properties of the asteroid populations.

Observational and Methodological Details

The NEOWISE survey employed a focused strategy through NEATM (Near-Earth Asteroid Thermal Model) to compute various attributes of asteroids, such as diameters and albedos from thermal IR data. A subset of 428 NEAs were thoroughly analyzed, providing a robust baseline for adjusting models related to NEO populations biases. The research capitalized on both the cryogenic phase (using all four bands) and the post-cryogenic phase (using bands W1 and W2) to ensure comprehensive coverage of NEO characteristics. The simulations for understanding the observed distribution were validated using a synthetic solar system model, with properties informed by current asteroid dynamics models.

Implications and Future Directions

The implications of this research are two-fold: practical and theoretical. Practically, understanding the size distribution helps in advancing projects focused on planetary defense. The identification of potentially hazardous asteroids with increased precision aids in mitigating impact threats over time. The research theoretically advances our understanding of the formation and evolution of NEOs by providing a clearer picture of their origins and physical characteristics than previously available.

Future developments may improve as additional observations are collected and integrated with this dataset, allowing for the further refinement of statistical models. Continued analysis could focus on extending this data to include detailed consideration of near-Earth comets and the examination of how these objects could impact the original paper's findings.

This paper significantly enriches the dataset available within the planetary science domain and sets a robust foundation for future research focusing on both observational techniques and dynamic modeling of NEOs.